In 1861, his bravery was noted when he attempted to rescue the men and boys confined in a coal-pit at Clay Cross during an inundation. In 1863, and again in 1864, he examined and reported on the Moselle coalfield, near Saarbriick. He wrote and delivered a paper on the local minefields to the Institute of Mechanical Engineers in Birmingham. He became a keen member of his local church starting a horticultural society and becoming a church warden.
On 12 December 1866, while at his house at Duffield, he learned that the Oaks Pit, near Barnsley, was on fire. Together with three others, including Mr Smith, an engineer and Mr D.Stewart, the steward of the colliery, he descended to make a complete exploration of the mine. They were one of the last parties to enter the mine; previous volunteers had been lost or had abandoned their rescues. One of the party returned to the surface to send down volunteers, but Jeffcock remained below directing such life-saving operations as could be carried on during the night of 12 December.
Before further help could arrive on the morning of the 13th, a second explosion killed Jeffcock and all but one of the whole band of 30 volunteers. The sole survivor was rescued on 14 December 1866 by Thomas William Embleton and John Edward Mannatt. In all 361 people were lost in the incident, including the 29 rescuers. The mine was sealed, and Jeffcock's body was not recovered until 5 October 1867, when it was buried in Ecclesfield churchyard.